1. Field of the Invention
The present invention relates to a nitrogen-doped graphene coated nano sulfur positive electrode composite material, a preparation method thereof, and an application of the composite material in lithium sulfur batteries, which relates to chemical power and material science fields.
2. Description of Related Art
The Lithium sulfur batteries, based on oxidation reduction reaction of double electron electrochemical, have been paid more and more attention by academia, industry and governments greatly, because of the advantages of their high theoretical specific capacity (1675 mAh/g, almost 10 times more than the theoretical specific capacity of the traditional LiFePO4 lithium battery), their high theoretical specific energy (2600 Wh/Kg), and the low price and low toxicity due to rich reserves of active substance sulfur.
The lithium sulfur battery device, being a chemical conversion battery, is composed of a sulfur positive electrode, an electrolyte, and a lithium negative electrode. Wherein an active material, a conductive agent and a binder with an appropriate mixing ratio are selected as the sulfur positive electrode. The factors, such as low conductivity and insulation of the sulfur in the active material and the polysulfide (Li2Sx, x=1˜8) from the charging and discharging products, the polysulfide (Li2Sx, x=4˜8) from the charge-discharge intermediate products easily dissolved in the electrolyte and then shuttling between the two electrodes, and the electrode structure damage as a result of the volume expansion (about 80%) caused when the lithium sulfide is completely converted from the electrochemical reaction of the active material sulfur, all cause the low utilization rate, the irreversible loss and the capacity loss of the active material, thereby causing the low actual capacity, and the poor cyclic performances and rate of the lithium sulfide battery, which seriously restrict the practical application of the battery.
In order to solve the problems in the lithium sulfur battery and to improve the device performance, the most effective method in the present research is to vulcanize the nano-active material and to load the material to the carbon based material (carbon nanotubes, porous carbon, graphene, carbon fiber, graphene oxide, etc) with good conductivity, thereby forming the composite positive electrode material. The utilization rate is improved by using the electrical conductivity of the carbon based material and the electrochemical activity of the low conductivity active substance sulfur realized by touching the nano sulfur. The discharge capacity and cyclic performances are increased by using the high specific surface area of these materials during the electrochemical process and the created negative effects occurred in the electrochemical process. The high specific surface area of these materials limits the polysulfide to be dissolved in the electrolyte during the electrochemical process. For example, the graphene is a single atomic layer carbon film composed of sp2 hybridized carbon atoms arranged in the hexagonal close packed structure. The graphene, having an excellent electrical conductivity, a good chemical stability, excellent mechanical properties and a high theoretical specific surface area (2630 m2g−1), is extremelysuitably used as the conductive carrier of the active material in the battery. At present, the graphene coated sulfur composites in the research (RSCAdvances, 2013, 3, 2558-2560; Nano Lett., 2011, 11, 2644-2647) shows better performance of the battery device. However, the capacity of these composite material under low rate is only 600˜800 mAh·g−1, the graphene materials under high rate does not show the advantages, which may be related to the reduction of surface area caused by the easy aggregation and stack of unfunctionalized graphene. The graphene oxide supported nano sulfur used as the positive electrode material of the lithium sulfur battery in the research (J.Am. Chem. Soc. 2011, 133, 18522-18525; Nano. Lett., 2013, 13, 5891-5899), shows a good cyclic stability. However, the reduction effect is not good when the graphene oxide is heated under the condition of a low temperature, the conductivity of the obtained composites is worse compared with the graphene, which needs 20 wt % of the conductive agent to be added additionally, thereby increasing the weight of the electrode and greatly reducing the energy density of the battery. The graphene coated sulfur/carbon nano fiber composites in the search (Nano Lett., 2013, 13, 2485-2489), includes the sulfur. The content of the sulfur is only 33%. In such a condition of the low sulfur content, the first discharging capacity of 0.1 C of the composites is only 1047 mAh·g−1 and has fallen to 700 mAh·g−1 after 50 cycles. The high rate discharge capacity is not prominent, and the capacity of 0.5 C is about 450 mAhg−1, the capacity of 1 C is about 400 mAh·g−1, and the capacity of 2 C is about 360 mAh·g−1. In addition, the graphene dispersion liquid used for coating in the method is made by the reduction of the toxic substance hydrazine as the reducing agent. In the research of multi walled carbon nanotubes coated by graphene/sulfur composite material (Nano Lett., 2013, 13, 4642-4649), the graphene in the composite material is made by heating and reducing the graphene oxide under 95° C. The reduction degree of the graphene oxide is finite. The conductivity of the gained composite becomes worse, thereby affecting the electrochemical property of the material, which is highlighted in the high rate performance. Although the first discharging capacity of 0.2 C reaches to 1396 mAh·g−1, its capacity of 1 C is 743 mAh·g−1, and its capacity of 2 C is 502 mAh·g−1. Therefore, it is urgent to develop the sulfur/carbon composite positive electrode material having the high specific capacity, the long cyclic life and the high rate performance with a friendly environment, a simple efficient process, and easy scaled production, and the preparing method of the same.